Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the production of alkyd resins. It is also a versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs.
In general, catalysts utilized for the oxidation of benzene and C.sub.4 hydrocarbons, such as butene, butane, and butadiene, to maleic anhydride are based upon vanadium and phosphorus. Various metal activators have been used to enhance the phosphorus-vanadium catalyst. The difficulty with the phosphorus-vanadium metal-promoted catalysts is that they tend to deactivate quite quickly. In this connection, U.S. Pat. Nos. 4,020,174, 4,094,816, and 4,089,807 teach that carbon tetrachloride can be used to reactivate the vanadium-phosphorus co-metal-promoted catalyst. In U.S. Pat. No. 3,296,282 and U.S. Pat. No. 3,474,041, there is described a method for the regeneration of vanadium-phosphorus oxidation catalysts used in the oxidation of olefins to make maleic anhydride. These references disclose the process of treating the catalyst with a phosphine, phosphite or phosphonate by periodically or continuously passing the phosphorus compound to the reactor, with or without interrupting the olefin feed flow. British Patent Specification No. 1,464,198 teaches regeneration of phosphorus complexes with certain phosphates. This reference does not disclose the reactivation of vanadium-phosphorus co-metal catalysts in the presence of about 1000 to about 40,000 parts per million of water in the feed gas stream as capable of being regenerated by organic phosphates nor does it suggest that the phosphates used in regeneration improved the color stability of the resulting maleic anhydride. Particularly, the reference does not appreciate that water in excess of 100,000 to 500,000 parts per million by weight in the feed gas stream is deleterious to the catalyst reactivation process.
We have now discovered a method for regenerating in situ vanadium-phosphorus co-metal catalyst complexes used in the vapor-phase oxidation of benzene and C.sub.4 hydrocarbons such as butane, butene and butadiene to maleic anhydride. According to our process, the vapor-phase oxidation of the C.sub.4 hydrocarbons and benzene to maleic anhydride is conducted by contacting the hydrocarbon feedstock in the presence of a vanadium-phosphorus-oxygen catalyst promoted with metals selected from the group consisting of zinc, molybdenum, tungsten, niobium, cobalt, tin, manganese, nickel, and uranium. The preferred co-metals are molybdenum and zinc. The catalyst is regenerated continuously or by batch method as desired during the vapor-phase oxidation of C.sub.4 hydrocarbons or benzene with an alkyl ester of orthophosphoric acid having the formula (RO).sub.3 P.dbd.O, wherein R is hydrogen or a C.sub.1 to C.sub.4 alkyl, at least one R being a C.sub.1 to C.sub.4 alkyl. The preferred method is to regenerate the catalyst continuously because significantly better maleic anhydride yields are obtained.
The catalyst to be reactivated can be prepared in various ways including the one disclosed in U.S. Pat. No. 3,862,146, issued Jan. 21, 1975, having Edward M. Boghosian as its inventor. The catalyst can also be prepared according to the process disclosed in U.S. Pat. Nos. 4,418,003, 4,416,802 and 4,416,803. Alternatively, the catalyst to be reactivated can suitably be prepared from an alcohol solution which has been reacted with phosphorus pentoxide and has been saturated with an inorganic acid, such as hydrogen chloride. Other ways to prepare the catalyst are disclosed in U.S. Pat. No. 4,328,126 wherein the catalyst is made from an organic solvent system.
Precipitation of the phosphorus-vanadium co-metal mixed oxide can suitably be effected by azeotropic distillation of the organic solvent and the water of reaction and the subsequent evaporation of the organic solvent. The atomic ratio of vanadium to phosphorus can suitably be in the range of about 0.5:1 to about 1.25:1, preferably in the range of about 0.6:1 to about 1:1. The total atomic ratio of zinc or molybdenum or the other co-metals to vanadium is advantageously in the range of about 0.005:1 to about 0.25:1. The atomic ratio of phosphorus to vanadium is suitably in the range of about 2:1 to about 0.8:1, preferably about 1:1 to about 1.7:1. The reactivation of the catalyst can also suitably be conducted by dissolving the alkyl ester of phosphoric acid in water and applying this solution in a uniform manner to the catalyst to be regenerated. This method is particularly suitable in continuous processes which utilize multi-tubular upflow reactors. In this process, the alkyl ester, in an aqueous medium comprising about 0.001 to about 90 wt% of the alkyl ester is sprayed as a liquid into the feed gas stream flowing to the reactor. This process has great advantages over conventional additions of regenerating agents, which entail plant shutdowns, since, in our novel process, the reactivation is conducted in situ without interrupting production or utilizing a hot oil vaporizer which tends to decompose alkyl phosphates. Our continuous process for color stabilizing maleic anhydride obtained by the vapor-phase oxidation of benzene or C.sub.4 hydrocarbons such as butane over a phosphorus-vanadium-oxygen catalyst promoted by a metal selected from the group consisting of zinc, molybdenum, niobium, tungsten, uranium, cobalt, and tin comprises regenerating the catalyst by contacting it during the vapor-phase oxidation with an alkyl ester of orthophosphoric acid having the formula (RO).sub.3 P.dbd.O where R is hydrogen or a C.sub.1 to C.sub.4 alkyl, at least one R being a C.sub.1 to C.sub.4 alkyl.
The continuous reactivation is applicable to phosphorus-vanadium catalysts and to phosphorus-vanadium catalysts promoted by metals, which are disclosed hereinabove. Suitable metals include molybdenum, zinc, tungsten, tin, cobalt, etc.
This invention also comprises a process for oxidizing benzene or C.sub.4 hydrocarbons such as butane, butene, and butadiene to maleic anhydride by contacting it in the presence of oxygen with the continuously reactivated catalyst in the presence of about 1000 to about 40,000 parts per million by weight of water based on the total weight of the feed gas stream and for improving the color and color stability of maleic anhydride produced by our novel process employing continuous or batch catalyst regeneration. Generally the amount of alkyl ester added is about 0.1 to about 100,000 parts per million by weight of the reactor feed gas stream. In a preferred novel process using continuous catalyst regeneration, the amount of alkyl phosphate added is in the range of about 0.1 to about 30 parts per million by weight of the reactor feed stream. Higher concentrations of of alkyl phosphate generally above about 30 parts per million by weight are useful in a batch catalyst regeneration process, preferably in a range of about 50 to about 100,000 parts per million by weight of reactor feed gas stream and more preferably about 1000 to about 100,000 parts per million by weight of reactor feed gas stream. The reactivation is conducted at a temperature of about 650.degree. to about 900.degree. F. The alkyl phosphate in a water medium comprising about 0.001 to about 90 weight percent, more preferably about 0.01 to about 50 weight percent, of the solution is contacted with the feed gas stream flowing to the reactor. If desired, the water and alkyl phosphate may be added separately to the feed gas stream instead of as a solution. Alternatively, the alkyl phosphate and water may be added directly to the butane feed prior to the mixing of the butane and air reactants. The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic mixtures of oxygen and diluent gases such as nitrogen may also be employed. Air enriched with oxygen may be used.
The gaseous feed stream to the oxidation reactors will normally contain air and about 0.2 to about 1.7 mole percent of the hydrocarbon such as benzene, butane, butene or butadiene. About 0.8 to about 1.5 mole percent of the hydrocarbon is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of the hydrocarbon feedstock, less than about one percent, of course, will reduce the total yield obtained at equivalent flow rates and, thus, are not normally employed for economic reasons. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but the preferred range of operations is at the rate of about 100 to about 4000 cc of feed per cc of catalyst per hour and more preferably about 1000 to about 2400 cc of feed per cc of catalyst per hour. Lower flow rates make the butane oxidation process uneconomical. A catalyst should be effective at flow rates of about 1200 to about 2400 cc of hydrocarbon feed per cc of catalyst per hour. There are catalysts which show good promise but when subjected to the hourly space velocity designated above show very poor yields. The amount of water added is about 1000 to about 40,000 parts per million by weight of the reactor feed gas stream. The preferred amount of water added is about 5000 to about 35,000 parts per million by weight of the reactor feed gas stream. Residence times of the gas stream will normally be less than about four seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury and at 0.degree. C.
A variety of reactors will be found to be useful and multiple tube heat exchanger-type reactors are quite satisfactory. The tops of such reactors may vary in diameter from about one-quarter inch to about three inches, and the length may be varied from about three to about ten or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the surface of the reactors at a relatively constant temperature and some medium is needed to conduct heat from the reactors, such as lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate, sodium nitrite, and potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metals surrounding the tube act as a temperature regulating body. As will be recognized by one skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes such as vycor, and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a preheat zone containing an inert material such as one-quarter-inch Alundum pellets, inert ceramic balls, nickel balls, or chips and the like present at about one-half to one-tenth the volume of the active catalyst present.
The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other medium is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than about 20.degree. to about 50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and hydrocarbon feedstock concentration.
The reaction may be conducted at atmospheric, superatmospheric or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to ensure a positive flow from the reactor. The pressure of the inert gases must be sufficiently higher to overcome the pressure drop through the reactor.
Maleic anhydride may be recovered by a number of ways well known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media, with specific operation and purification of the maleic anhydride. The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these examples are given for illustrative purposes only and should not be interpreted as limiting the invention in any way. In the examples, the terms "conversion", "selectivity" and "yield" are defined as follows: